The present invention relates generally to sheet feeding and separating apparatus and more particularly to a retard member for use in a friction retard sheet separator feeder which has a stable coefficient of friction, a stable feed angle and provides a very quiet sheet separation for a variety of different weight sheets. It has a particular application in the feeding of sheets in electrostatographic printing machines such as, for example, those illustrated in U.S. Pat. No. 4,660,963 to Stemmle.
The development of electrostatographic printing machines has brought about the need for simple, yet reliable, sheet separator feeder apparatus capable of handling sheets varying in length, thickness, weight and surface conditions. One of the more common arrangements involves friction retard feeders wherein separation and feeding is dependent upon a differential friction principle. In one such type of feeder, a feed roller surface has a relatively high coefficient of friction with paper while the retarding surface which may also be a roller driven in the opposite direction or alternatively a stationary pad having a coefficient of friction with paper less than that of the feed roller, but greater than that between two successive sheets of paper. In these feeders, the coefficient of friction of the feed roller with the paper must exceed the coefficient of friction of the retard member which must always exceed that of the coefficient of friction between two sheets of paper. In these separator feeders the region of contact between the retarding member and the feed roll forms a sheet queuing throat which is able to fan out or que sheets in the throat for feeding single sheets through the throat.
FIG. 1 schematically illustrates a typical sheet separator feeder apparatus capable of handling sheets varying in length, thickness, weight, and surface conditions which includes a sheet support platform 10 urged upwardly by spring 11 to advance sheets to be separated and fed to the friction retard nip formed between the retard member 12 and feed roll 13 The feed roller surface has a relatively high coefficient friction with the paper while the retard member has a lower coefficient of friction with the paper but its coefficient of friction with the paper is greater than the coefficient of friction between two successive sheets. This relationship and geometry enables the shingling or staggering of individual sheets into the nip between the feed roll and retard pad to the path defined by the sheet guide 15. Typically the feed roll is made from a silicone rubber or other elastomer having a coefficient of friction greater than about 1.5. While capable of performing satisfactorily, there are problems associated with these types of feeders. One of the more common problems is feeding reliability, that is the feeding of single sheets only from the nip between the feed roller and the retard member rather than a multi-feed of from two up to perhaps six sheets The multi-feed difficulty can be further appreciated with reference to FIG. 2, wherein a friction retard sheet separator feeder is provided which has a rigid and fixed sheet entrance guide to guide sheets into the sheet retard nip area. The multi-sheet feed situation occurs in the entrance guide area A in advance of the retard nip area B by a slug, six or more sheets, entering the entrance guide area which become pinched between the entrance guide and the feed wheel. Because the entrance guide is rigid and fixed, an additional normal force N is created between the entrance guide and the feed wheel, which creates a increased driving force to drive the slug through the retard nip creating a situation that overpowers the retard systems ability to frictionally separate the slug. In addition, lead edge damage from sheet stubbing is caused by the relatively large distance between the end of entrance guide area A and the entrance to the retard nip area B.
While capable of performing in an acceptable manner this feeder frequently experiences misfeeds and multi-feeds as a result of big changes in the feed angle which result from small changes in the feed roll diameter or the retard pad thickness. Accordingly, it is desirable to provide a greater stability in the feed angle in this retard paper feeder to provide increased stability.
Our U.S. Pat. No. 5,163,668 describes a retard pad assembly with a movable compliant entrance guide which addresses these issues. One approach to improving the stability of the feed angle in the system described in U.S. Pat. No. 5,163,668 is to relocate the pivot point of the retard pad assembly to a point quite remote from that illustrated in U.S. Pat. No. 5,163,668 wherein the retard assembly is mounted in a frame which is pivotally mounted to a pivot point on the retard assembly and urged upwardly toward the front edge of the retard assembly against a separator feed roll by a spring. However, with the relocation of the pivot point from the retard assembly mount to a location which is quite remote, additional difficulties are frequently encountered in that a highly objectionable noise is created which resembles the scratching of chalk or a fingernail on a blackboard between the retard member support member and the pivoting support frame. Since the friction retarding surface is typically an elastomer to be discussed in greater detail hereinafter, as the sheets of the paper are fed across the elastomer, the frictional relationship between the elastomer and the paper tends to stretch the elastomer until it reaches a limit at which time the coefficient of friction between the paper and the elastomer drops dramatically. If the coefficient of friction decreases with relative velocity the stretching force will decrease below an equilibrium force when slippage occurs. This causes oscillation of the elastomer known as stick-slip. This oscillation excites the paper like a loudspeaker.